The present invention relates to a wafer carrier positioning structure having a precise positioning accuracy.
A wafer carrier positioning structure as illustrated in FIGS. 22, 23 is known (for example, refer to paragraph 0028-0045 and FIGS. 2, 4, JP H11-168136A, and paragraph 0017-0058 and FIG. 1, JP 2005-162263A).
Such a wafer carrier positioning structure includes a carrier body 1 and a lid. The carrier body 1 substantially having a box shape includes at one side thereof an opening 1a through which a semiconductor wafer W as a wafer can be placed in and taken out. The lid opens and closes the opening 1a of the carrier body 1.
The carrier body 1 includes inside thereof a plurality of aligned ribs 1c and 1a bottom face 1b. The bottom face 1b includes three pairs of positioning ribs each having a pair of positioning ribs 3, 3 disposed to be parallel to each other in the radial direction of the semiconductor wafer W. The three pairs of positioning ribs are disposed in the right and left portions of the bottom face 1b on the opening 1a side and the center portion of the peripheral portion of bottom face 1b on the side opposite the opening 1a, so as to be located substantially in the vicinity of apexes of a triangle shape in plan view.
The lower end of each of the positioning ribs 3, 3 includes an inclined plane 3a. 
The inclined planes 3a, 3a are formed to oppose each other. In the case of mounting the wafer carrier onto an automatic wafer transfer device 2, each of kinematic couplings 2a as a positioning pin projecting from a mounting face 2b of the device 2 has contact with each pair of the inclined faces 3a, 3a, so as to perform the positioning of the wafer carrier.
The bottom face 1b of the carrier body 1 includes a bottom plate 4 as a guide member. The bottom plate 4 is detachably attached onto the bottom face 1b of the carrier body 1 by externally fitting each of guide grooves 4a provided on the bottom plate 4 to each pair of positioning ribs 3, 3.
The lower of each of the guide grooves 4a includes inclined faces 4b, 4b each having an inclined angle which follows the inclined angle of the inclined face 3a. 
Next, the operations of the above wafer carrier positioning structure will be explained.
In this wafer carrier positioning structure, when attaching the bottom plate 4 onto the bottom face 1b of the carrier body 1, each of the guide grooves 4a of the bottom plate 4 is externally fitted to each pair of the positioning ribs 3, 3 projecting from the bottom face 1b of the carrier body 1,
As described above, the inclined planes 4b, 4b formed in the lower peripheral part of the guide groove 4a of the bottom plate 4 have inclined angles which follow the inclined angles of the inclined faces 3a, 3a of the positioning ribs 3, 3.
Accordingly, as illustrated in FIG. 23, when mounting the carrier body 1 onto the device 2, the kinematic couplings 2a are smoothly guided by the inclined faces 4b, 4b; thereby, the kinematic couplings 2a can be positioned below the inclined faces 3a, 3a. 
With the carrier body 1 is located in a regular mounting position of the device 2, the semiconductor wafers W can be precisely placed in and taken out via the opening 1a by means of a robot of an automatic device, without damaging the wafer.
A wafer carrier positioning structure as shown in FIG. 24 is also known (for example, refer to paragraph 0008-0014 and FIGS. 1, 3, JP 2003-524550A).
In this wafer carrier positioning structure, a bottom plate 5 includes a plurality of boss holes 5a. Each of the boss holes 5a is externally fitted to each of boss portions projecting from the bottom face 1b of the carrier body 1.
The bottom plate 5 includes a retaining feature 5b which engages with a clamp portion of the device 2.
The retaining feature 5b is used for mounting the carrier body 1 onto the device 2 and also demounting the carrier body 1 from the device 2. For example, in the case of mounting the carrier body 1 onto the device 2, the clamp portion of the device 2 engages with the retaining feature 5b. 
In addition, a wafer carrier positioning structure, which directly integrates a portion corresponding to the above guide member into the bottom face 1b of the carrier body 1 by means of welding or a screw, is known (for example, refer to paragraph 0016-0036 and FIG. 1, JP 2002-353299A).
However, in the above wafer carrier positioning structure shown in FIG. 22, 23, each of the kinematic couplings 2a has contact with the inclined planes 3a, 3a formed in the lower ends of the positioning ribs 3, 3, so as to support the weight of the carrier body 1 from underneath.
For this reason, there is a problem in that the entire carrier body 1 needs to be changed if the positioning ribs 3, 3 are damaged by the contact of the kinematic couplings 2a. 
Since the carrier body 1 houses the semiconductor wafers W, the carrier body 1 requires a structure which is made of a relatively expensive highly-pure resin material and also has an antistatic function. Therefore, it is expensive to change the entire carrier body 1.
Moreover, as illustrated in FIG. 24, in the wafer carrier positioning structure, which fits the boss portions provided in the bottom face 1b of the carrier body 1 into the boss holes 5a formed in the bottom plate 5, so as to perform the positioning, the bottom plate 5 can be reliably fastened to the bottom face 1b of the carrier body 1 by tightly fitting the boss portions into the boss holes 5a, while improving the positional accuracy in the directions along the bottom face 1b. However, when attaching the bottom plate 5 onto the bottom face 1b and also detaching the bottom plate 5 from the bottom face 1b, the working property may be decreased, and also when exceeding the tolerance, the bottom plate 5 may be damaged.
Furthermore, in a method for visually confirming a fitting degree, it is difficult to confirm whether the bottom plate 5 is reliably fastened in the inward and outward direction (vertical direction) of the bottom face 1b. 
In the wafer carrier positioning structure, which integrates the guide member into the bottom face of the carrier body by means of a screw, the portion corresponding to the guide member may be damaged by the contact of the kinematic couplings 2a. In the case of mounting the carrier body 1 onto the device 2 and demounting the carrier body 1 from the device 2, if the retaining feature 5b is deformed, worn away or damaged, it is necessary to change the entire carrier body 1 or unfasten the screw to fasten a new guide member onto the bottom face of the carrier body.
As described above, in the case of attaching the new guide member, it is necessary to reliably locate the guide member on the bottom face of the carrier body, so as to fasten the guide member onto the bottom face. For this reason, the number of working steps required for changing the guide member is increased. Thus the costs for changing the guide member are increased.
Accordingly, there is a need for a wafer carrier positioning structure capable of easily attaching a guide member to an exact position and also capable of using a carrier body at low cost by changing only a guide member.